Chalcogenide glass nanospheres with tunable morphology by liquid-phase template approach

Summary Chalcogenide glass (ChG) with unique material properties has been widely used in mid-infrared. Traditional ChG microspheres/nanospheres preparation usually uses a high-temperature melting method, in which it is difficult to accurately control the size and the morphology of the nanospheres. Here, we produce nanoscale-uniform (200–500 nm), morphology-tunable, and arrangement-orderly ChG nanospheres from the inverse-opal photonic crystal (IOPC) template by the liquid-phase template (LPT) method. Moreover, we refer to the formation mechanism of nanosphere morphology as the evaporation-driven self-assembly of colloidal dispersion nanodroplets within the immobilized template and find that the concentration of ChG solution and the pore size of IOPC are the key to control the morphology of the nanospheres. The LPT method is also applied to the two-dimensional microstructure/nanostructure. This work provides an efficient and low-cost strategy for the preparation of multisize ChG nanospheres with tunable morphology and is expected to find various applications in mid-infrared, optoelectronic devices.


INTRODUCTION
Common microspheres/nanospheres often exhibit unique properties and are widely used in areas such as optical devices, drug delivery, and biochemical catalysis, benefiting from their limited dimension and the flexible preparation methods, as well as their tailored morphologies and chemical compositions. [1][2][3][4] Meanwhile, among the mid-infrared materials, chalcogenide glass (ChG) has become an ideal material for manufacturing new micro-nano photonic devices, due to their excellent mid-to-far infrared transmission performance, high refractive index, nonlinear refractive index, ultrafast nonlinear optical response, etc. [5][6][7] ChG microspheres/nanospheres are usually prepared by melting method, in which process the ChG preforms (powder or fiber, etc.) turn into a molten state after reaching the glass transition temperature and a regular spherical shape is formed driven by the surface tension/Rayleigh instability. [8][9][10] The melting method has certain limitations: 1) it is difficult to prepare the microspheres/nanospheres with complex architecture (e.g., porous microspheres/nanospheres) and 2) it often requires a high-temperature environment. The liquid-phase method is frequently used to realize complex architecture in microspheres/nanospheres, in which the particles are produced through nucleation, co-precipitation or sol-gel, etc. 11 However, it still has problems such as low controllability over nanospheres size, complex preparation process, etc. Another useful method, called template method, 12 could use polymer templates to prepare nanospheres of different materials (metals, polymers, and oxides) 13 or use patterned template to produce nanostructure with different shapes. 14 This method has advantages on the versatile preparation process, controllability over size and shape, and wide material compatibility.
In this article, we use a method, called liquid-phase template (LPT), which combines the merits of liquidphase method and template method to produce uniform ChG nanospheres. In this LPT method, the ChG solution is filled into the IOPC template which has three-dimensional uniform pores, and after the solvent is completely evaporated, the ChG nanospheres with uniform size are obtained. The nanospheres with tunable morphology could be prepared using this method by tuning the ChG solution concentration and the template pore diameter. The LPT method is also versatile, as ChG ''nano-bowls'' and ChG micro-discs could be produced in different templates.
The process of LPT method includes preparing the IOPC template, filling the ChG solution, evaporating the solvent, and removing the template ( Figures 1A-1D). Specifically, evaporation co-assembly is performed to prepare the opal photonic crystal (OPC) film, 15 which consists of polystyrene nanoparticles (PS-NPs) opal film and silica gel matrix material distributed uniformly. The scanning electron microscopy (SEM) images of PS-NPs film show the highly ordered structure ( Figure 1E). After high-temperature sintering, the IOPC SiO 2 template is obtained ( Figure 1F), with the uniform pore size inside determined by PS-NPs. Owing to the high adhesion to the substrate and exceptional mechanical performance which are further improved by the annealing process, this template is able to keep its integrity during the solution-filling process. 16 Subsequently, the bulk ChG (As 30 S 70 ) is dissolved in highly volatile amines (N-propylamine) forming nano-colloidal solutions 17 and filled into the IOPC template by spin coating and ultrasonic oscillation method. Next, the IOPC template is heated for 1h at 80 C to evaporate the amines solvent and annealed at 120 C for 6 h in vacuum to make sure the amines solvent is totally evaporated. After the annealing process, the ChG nanospheres OPC is formed inside the IOPC template ( Figure S1). Finally, the IOPC template is etched off in hydrofluoric acid, and a large area of uniform ChG nanospheres OPC film is obtained, which shows different morphologies ( Figures 1G and 1H). Owing to the connection among the ChG nanospheres ( Figure S2) and the ChG being generally inert to acids, the ChG nanospheres remain on the substrate and keep the OPC structure. 18,19 Characterization of the ChG nanospheres The energy-dispersive spectrometer (EDS) analysis shows that the elemental composition of nanospheres prepared by the LPT method is As and S, which is consistent with that of bulk glass (Figure 2A). The Raman spectrum is collected after the ChG nanosphere OPC film gets irradiated by the 473 nm laser ( Figure 2B). It has a wide Raman vibration band at 350 cm À1 , which belongs to the As-S bond and further confirms the nanosphere composition. In addition, the spectrum has a peak at 495 cm À1 which hints that nanospheres have a composition close to As 36 S 64 , instead of As 30 S 70 in the bulk glass. 20 The loss of sulfur could happen during the dissolving process, in which some sulfur precipitates out of the solution due to the dissolving limit. 21 The X-ray photoelectron spectroscopy (XPS) characterization results ( Figure 2C) also confirm that the composition of nanospheres is As and S and show an unnecessary O1s peak. The reason for this may be that the sulfur gets oxidized in the heating and annealing process and brings in the O element. The X-ray diffraction (XRD) confirms that the nanospheres are amorphous after being annealed at 120 C  Figure 1G inset shows the optical picture of the film. Figure 1H inset shows the TEM image displaying the roundness of the nanospheres. iScience Article for 6 h ( Figure 2D). We also characterize the reflection spectra of the ChG OPC films ( Figure S3). The reflection peak exhibits blue-shift trend with increasing incident angle, which is consistent with the properties of photonic crystals. More importantly, the reflection peak could be tuned by changing the size of the nanospheres and ChG solution mass fraction. This photonic property makes them suited to light relevant applications, such as sensing, lasers, energy storage, and so forth. For example, simply altering the refractive index contrast by changing one of the materials with a material of different refractive index will produce a shift in the stop band of the photonic structure, which is ideal for sensor applications to monitor the temperature, detect the gas, etc. 22,23 ; by embedding an optical gain medium (dye molecules or quantum dots) in the three-dimensional structures, a high refraction index contrast between ChG nanospheres and air can realize low-threshold lasing at the band edge of photonic crystals, and the lasing wavelength can be controlled by simply changing the size of the nanospheres. 24 An important feature of the LPT method is that ChG nanospheres with different morphologies could be flexibly prepared by adjusting the concentration of the ChG precursor solution and the pore size of the IOPC template. Figure 3 shows the nanospheres morphology diagram under different IOPC template pore sizes and ChG mass fractions. At a low ChG solution mass fraction (13.91 wt %), the nanospheres are mostly semisphere or hemishell under any of the template pore sizes, and the OPC structure of the ChG nanospheres film is mostly broken (Figures 3A-3D). As the mass fraction increases to 27.82 wt %, ''large holes'' appear on the surface of the ChG nanospheres, which correspond to the pore interconnectivity points in the IOPC template ( Figures 3E-3H). When the mass fraction is further increased to 55.63 wt %, the ''large holes'' disappear and the complete ChG nanospheres are fabricated (Figures 3I-3L). The cross-section image of the nanospheres OPC film shows a consistent result ( Figure S4). In the meanwhile, comparing sphere size over pore size, it is not difficult to find that the size of the ChG nanospheres gradually increases along with the pores. It is worth noting that when produced from the same template, the nanospheres prepared at high mass fraction are smaller than those prepared at low mass fraction, which hints that the inside structure of nanospheres prepared at low mass fraction is loose and porous ( Figure S6). This unique structure makes the nanospheres possess large surface area, low density, and high loading capacity. These properties enable nanospheres to have a large variety of applications, including microcontainers/nanocontainers, environmental remediation, biomedicine, and more. For example, void space existing inside or on the porous spheres can provide the "microenvironment" for iScience Article many chemicals such as dye molecules, organic drugs, and inorganic nanoparticles, enabling controlled molecule release or drug delivery; 25,26 owing to their large surface area, porous nanospheres have strong affinity toward dyes, organic pollutants, heavy metal ions, etc., making them ideal for applications of water purification and environmental remediation. 27

Discussion of ChG nanospheres-forming mechanism
Based on the phenomenon that different solution concentrations and pore sizes of the template could prepare tunable morphology, the ChG nanospheres-forming mechanism is discussed. Figure 4A shows the two critical steps of making ChG nanospheres: 1) ChG solution filling into the template and 2) ChG solvent being evaporated to get the ChG nanospheres. During the ChG solution filling process, the IOPC template ensures the filling rate of ChG solution with up to 12 interconnected points between neighboring pores. To further increase the filling rate, different methods (spin coating, ultrasonic oscillation, etc.) are used to get a better filling quality ( Figure S5).
During the solvent evaporation process, the ChG solution is gradually broken into small nanodroplets, 28 whose size are limited by the pores of the template. The subsequent evaporation process of the iScience Article nanodroplets can be referred to as the evaporation-driven self-assembly of colloidal dispersion nanodroplets within the immobilized template. Figure 4C illustrates the process of ChG nanodroplets turning into ChG nanospheres: as the solvent is evaporated, the nanoclusters (average 1-4.5 nm) in the nanodroplets aggregate to form nanospheres with porous structure and tunable morphology. 21,29,30 Figure 4B demonstrates the porous structure inside the ChG nanospheres due to the gas generated by the chemical reaction during the evaporation process. 31 The concentration of the ChG solution and the pore size of the template play an important role in tuning the ChG nanosphere morphology, and Figure 4C illustrates the evaporation-driven self-assembly of colloidal dispersion nanodroplets within the immobilized template in three cases and fully agrees with the experimental phenomenon in Figure 3. The nanodroplets initially behave like pure liquids and shrink isotropically during evaporation but eventually form a viscoelastic shell of dense nanoclusters on their surface. 32 This is due to a thermophoretic force that originates from the temperature gradient at the droplet surface and thus also causes the concentration gradient of the solute in the droplet. 33 When using low-concentration ChG solutions, the nanoclusters on the nanodroplet surface are dominated by the concentration gradient force pointing to the sphere center due to the solute concentration gradient. In addition, they are also subjected to the outward electrostatic repulsion force provided by the adjacent surface clusters and the inner nanoclusters of the nanodroplets. For the nanocluster located at the pore junction of the template, the electrostatic repulsion force is not enough to balance with the concentration gradient force, resulting in collapse. The nanoclusters not at the pore junction of the template are usually subjected to the outward adhesion force from the template, which is more stable than the nanoclusters at the pore junction of the template. Therefore, the nanodroplets formed hollow spherical shells with large holes at the pore junction of the template (Figures 3A-3H). In the case of high-concentration ChG solution, the concentration gradient force toward the sphere center of the surface nanoclusters due to the concentration gradient of the solute will be relatively reduced, and the electrostatic force provided by the nanoclusters inside the nanodroplet will be larger because of the inherent constraints of availability of space. 32 In the largesized template pore, the nanoclusters located at the pore junction of the template also occasionally iScience Article collapsed at the early stage of solvent evaporation, resulting in shallow pits on the surface of the prepared nanospheres ( Figure 3L). While in the small-sized template pore, the concentration gradient force toward the sphere center of the surface nanoclusters further reduced, the decrease of the nanodroplet radius increases the outward electrostatic repulsion provided by the adjacent surface and the inner nanoclusters of the nanodroplet. Therefore, the surface nanoclusters maintain the quasi-steady state during the whole solvent evaporation process, and the smooth surface nanospheres are finally prepared ( Figure 3I). The evaporation-driven self-assembly of colloidal dispersion nanodroplets is relatively more stable and slower when using a high-concentration ChG solution. Therefore, the diameter of the nanospheres prepared with a high-concentration of ChG solution is slightly smaller than the spherical shells prepared with a low concentration of ChG solution ( Figure S6). The detailed analysis of the evaporation-driven self-assembly of colloidal dispersion nanodroplets within the immobilized template is in method details.

Process versatility
The LPT method could be extended to prepare ChG with other microstructures/nanostructures. For example, on the top of the ChG nanospheres OPC film, the "bowl-like" nanostructure is formed because the uppermost layer of the IOPC template is a semi-enclosed structure; the ''bowl-like'' ChG nanostructure has a diameter of around 150 nm ( Figures 5A and 5B). The LPT method is also used with a copper mesh to produce ChG micro-discs. The copper mesh, as the template in this method, is suspended, and the ChG solution is dripped on top of it ( Figure S7). After the solvent is completely evaporated, ChG discs with diameter of around 60 mm are obtained by etching off the copper mesh with hydrochloric acid (Figures 5C  and 5D).
In summary, we demonstrate an LPT method to realize ChG nanospheres with tunable size and morphology. We discuss the ChG nanosphere-forming mechanism and propose a model to explain the morphology of the nanospheres. This method is suitable for not only three-dimensional templates (such as IOPC) but also two-dimensional templates (such as copper mesh). This work provides an efficient and iScience Article low-cost strategy for the preparation of multisize ChG nanospheres with tunable morphology and is expected to find a variety of applications in mid-infrared, optoelectronic devices.

Limitations of the study
ChG microspheres of different components have not been prepared, and the universality of the LPT method has not been verified. In addition, when preparing ChG nanospheres from low concentration of ChG solutions, the surfaces are often covered with a thin ChG film, which has a certain impact on the expression of photonic crystal properties.

STAR+METHODS
Detailed methods are provided in the online version of this paper and include the following: